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- Map size
- Make an image twice the size of the primary beam (e.g.
at 90 GHz and
at 230 GHz for PdBI antenna)
to ensure that all the area of the primary beam (inner quarter of the
dirty map) will be cleaned whatever the deconvolution algorithm is used.
However, avoid making a too large dirty image because the CLEAN
algorithms will then try to deconvolve region outside the primary beam
area where the noise dominates.
- Support
- Start your first deconvolution without any support to
avoid biasing your clean image. If the source is spatially bounded, you
can define a support around the source and restart the deconvolution with
this a priori information. Be careful to check that there is no
low signal-to-noise extended structure that could contain a large
fraction of the source flux outside your support... Avoid defining a
support too close to the natural edges of your source. Indeed,
deconvolving noisy regions around your source is advisable because it
ensures that you do not biased your deconvolution.
- Stopping criterion
- Choose the right stopping criterion.
- Estimate an empirical noise on your first deconvolved cleaned image
with GO NOISE.
- If this empirical noise value is larger the value computed from the
visibility weights (This noise value is one of the output of the
UV_MAP command), your observation is probably dynamic limited,
i.e. you have a bright source whose dirty sidelobes are much larger
that the thermal noise. In this case use, set ARES to 0 and
FRES to a fraction which depends on the sidelobe level of your
dirty beam.
- Else you are in the noise limited case. Set FRES to 0 and
ARES to a fraction of the empirical noise value (typically
0.5).
- Convergence checks
- Ensure that your deconvolution converge by
checking that
- The cumulative flux as a function of the number of clean component
has reached a roughly constant level (use
\FLUX
option of the
deconvolution commands to see this curves).
- The residual map looks like noise where the source appears in the
clean map.
Else change the values of the stopping criterion, in particular the
number of clean components (NITER).
- Deconvolution methods
- If you want a robust result in all cases,
start with HOGBOM. If you prefer obtaining a quick result, use
CLARK but you then first need to check that the dirty sidelobes are
not too large on the dirty beam. If you obtain stripes in your clean
image:
- First check that there is no spurious visibilities that should be
flagged.
- Then check that your deconvolution converged.
- If it is clear that you have an extended source structure, you
should first ask yourself whether you are in the wide-field imaging
case and act accordingly (see next chapter). Else you can try a
CLEAN variant which better deals with cases that implies a large
spatial dynamic. This is rare at PdBI.
- Outside help
- Always consult an expert until you become one.
Next: Wide-field imaging and deconvolution
Up: Comparison and practical advices
Previous: Comparison of deconvolution algorithms
Contents
Index
Gildas manager
2014-07-01